Speech-controlled bilateral amplifier

ABSTRACT

A speech-controlled bidirectional amplifier that includes first and second transmission channels each with a control member controlled by speech signals derived at control points preceding the control member. First and second comparison devices responsive to the levels of the speech signals control a third comparison device that in turn controls the attenuation level of the control members. The amplifier is arranged so that in the rest condition both control members provide a high signal attenuation. When speech signals are transmitted through one channel, the other channel is cut-off. However, an interrupt facility is provided whereby a speech signal above a given level in the cut-off channel will cause the apparatus to switch over so that the cut-off channel is opened and the previously open channel is then cut-off.

I United States Patent [151 3,660,603

Andersen 1 1 May 2, 1972 [54] SPEECH-CONTROLLED BILATERAL FOREIGNPATENTS OR APPLICATIONS AMPLIFIER 668,858 4/1949 ore-d1 1111121111..179/1 vc [72} Inventor: Bjorn Andersen Oslo Norway 1,281,499 3/1966Germany ..179/170.6

[73] Assignee: U.S. Philips Corporation, New York, NY. p i E \,aminerKathleen Cl ff [22] Filed: Aug. 28, 1969 Assistant Examiner-Jon BradfordLeaheey Attorney-Frank R. Trifari [21] Appl. No.: 853,881

[57] ABSTRACT [30] Foreign Application Priority Data A speech-controlledbidirectional amplifier that includes first Aug. 29, 1968 Norway..3369/68 an econd transmission channels each with a control member 7controlled by speech signals derived at control points preced- [52] US.Cl. ..179/1 VC, 179/1 H ing the control member. First and secondcomparison devices [51] Int. Cl. ..H03g 3/24 responsive to the levels ofthe speech signals control a third Field of Search 1 VC, 1 F5, 170 R,comparison device that in turn controls the attenuation level 179/170 B,1 31 170-6 of the control members. The amplifier is arranged so that inthe rest condition both control members provide a high signal [5 6]References and attenuation. When speech signals are transmitted throughone UNITED STATES PATENTS channel, the other channel is cut-off.However, an interrupt facility is provided whereby a speech signal abovea given level 10751045 1/1963 Clemency 179/1 HF in the cut-off channelwill cause the apparatus to switch over 3,113,181 12/1963 Soderbaum..179/l70 so that the cut-off channel is opened and the previously openchannel is then cut-off.

9 Claims, 1 Drawing Figure DEVICE FR CONTROL AWL DEVICE AMPL. SPEAKER M1, R 1 1 1 1 r 1 1 I 1 1 1 AMPL. 1 r PR2 1 1 RECTIFIER, l 1 ND 1 i 1 a$2 2 1 1 1 1 1 GATES D I l a 115 l N0 0 g COMPARISON I 1 l 1 1 1 1 1 ll- PATENTEIIIIII 2 I972 3. 660,603

CONTROL AMP! DEVICE AMPL. SPEAKER I I R I 2 HI r'b I I I I I I i AM L.AMP I 5 1 my I I RE TIFIER 4 RECTIFIER I DELAY I M I DB9 I I H Q I I I0A1 51 g GATE K I I D2 I I I I B E3 m I 2 I I C2 S2 0A2 I COMPARISON IDEVICE I I l PR1 FKZ I I l I I I l I I l INVENTOR. BJORN ANDERSE NSPEECH-CONTROLLED. BILATERAL AMPLIFIER The present invention relates toa speech-controlled bilateral amplifier in which each amplifying channelincludes a control member governed by a control circuit to which speechsignals are applied that are taken from the two amplifying channels atpoints preceding the control member of each channel. I

Such amplifiers may be employed with loud-speaking telephones orso-called intercom systems which allow a conversation in two directions.The inputs of the amplifiers are connected to microphones and the outputto loud speakers, the microphone of one channel being located in thesame room as the loudspeaker of the other channel and conversely. Ingeneral it is required to arrange the microphone and the loudspeaker inthe same cabinet so that they are positioned relatively close together.A very strong acoustic coupling between the two elements is thenunavoidable.

This implies the risk .of acoustic feedback. In order to avoid same itis common practice to have the amplifier controlled by the speechsignals so that the channel in use has a high amplification and theother channel is cut off or at least has a sufficiently lowamplification.

Of course, this method of control has to be performed rapidly andsmoothly and apart therefrom such systems have to satisfy various otherrequirements.

When one of the parties starts speaking the associated amplifyingchannel has to open rapidly, even for a low speech level, so that eventhe beginning of the first word will not be lost. During shortintervalsbetween words or sentences of the conversation it should not bepossible for the direction of transmission of the amplifier to beautomatically reversed due to reverberation or cross-talk between theloudspeaker and microphone in the other'room.

It is desirable for theloudspeaker to reproduce the sound with adequatevolume, for example, at the normal speech level, whereas at any instantthe other-party should be able to interrupt .the first one andto reversethe direction of transmission of the amplifier without theneed forraising his voice markedly. The reversal of the channelsshould beperformed rapidly so that each party is able'to make a remark during thecontinuous flow of words of the other party without the beginning oftheremark being suppressed due to inertia of the interchange. In addition,the interchange of the channels should not give rise to instability.

lt isparticularly important thatthe persons speakingshould be free tomove about inthe roomsso that they need not always be near themicrophone-loudspeaker combination.

Known devicesdomot satisfy all of said requirements or satisfy them onlyto alimited extent because great difficulties arise from the fact thatvarious requirements are to some extent in conflict with each other.

In general a loudspeaker is located nearer to the microphone than theperson talking into it. When the loudspeaker reproduces the sound atspeech level,-the associated microphone which, picks up the cross-talksignal can obviously provide a much stronger signalthan the microphoneto which the direct speech signal is. applied.

In systems in whichspeech signals for controlling the amplification arederived in each channel at a point preceding the control members and inwhich .the channel to which the strongermicrophone signal is applieddominates and is driven to full amplification whereas the other channelis cut off, it is not possible, dueto the strong cross-talk between theloudspeaker and the microphone, to obtain a high acoustic amplification,that is to say, a strong sound reproduction, and moreover the partieshave to remain near the microphones.

in known devicesin which the speech control signals are derived at apoint beyond the control members, a strong sound reproduction can beobtainedybut it is very difficult to interrupt the conversation exceptin-the case of long pauses in the other partys speech.

The invention provides a; particularly efficacious solution of theseproblems.

The invention is characterized in that the speech signals from eachchannel are applied each through a first signal circuit to a first inputof two comparison devices and the speech signals from each channel arefurthermore each supplied through a second signal circuit to a secondinput of the two comparison devices. The amplification of the first twosignal circuits exceeds that of the second signal circuits and thecomparison devices are designed so that they supply an output worksignal when the signal level at the first input is higher than that atthe second input. The output signals are fed to an input of a first anda second gate circuit, respectively, in a third comparison device, whilethe outputs of the gate circuits are coupled with a second input of theother gate circuit in a manner such that, when a work signal is fed toone of the firstmentioned inputs of one of the gate circuits the othergate circuit is cut ofi". The control voltages for the two controlmembers are taken from the outputs of the gate circuits of the thirdcomparison device.

The invention will be described more fully with reference to a singlepreferred embodiment shown schematically in the sole FIGURE of theaccompanying drawing.

The amplifier comprises two channels. In one channel the speech signalof the microphone M, is applied through the microphone preamplifier T,,auxiliary control member DR,, the control member R, and the outputamplifier U, to the loudspeaker H,. The other channel is arranged in anidentical manner and comprises the preamplifier T,, auxiliary controlmember DR,, the control member R, and the output amplifier U Theauxiliary control members DR, and DR, may be omitted since they are notessential to the invention. The purpose and function thereof will bedescribed in detail below. The microphone M, and the loudspeaker H, arelocated in the same room, as are the microphone M, and the loudspeakerH,. The speech signals for the control circuit are derived in thechannels at points P, and P, preceding the control members R, and R,,respectively, so that these signals are not affected by the randomstates of the control members R, and R,

The signal at point P, is applied through a first signal circuitincluding an amplifier FK,, a rectifier DA, and a delay device S, to theinput A, of a voltage comparison device K,. The signal at point P, isapplied similarly through the amplifier FK,, the rectifier DA, and thedelay device S, to the input A, of a second comparison device KMoreover, the signals at points P, and P, are each applied through asecond signal circuit including the amplifier FR,-and the rectifier D8,,and the amplifier PR, and the rectifier DB,, respectively, to a secondinput B and B,, respectively, of the comparison devices K, and K,,respectively.

The comparison devices K, and K, are arranged in known manner so thatthey supply an output work signal, for example, a high output voltage,when the signal level at their first input A, add A, respectively,exceeds by a given threshold value the level at the second inputs B, andB,, respectively.

The outputs of the comparison devices K, and K, are connected to theinputs C, and C, of the gates ND, and ND, of a third comparison device.The outputs of the gates ND, and ND, are each connected to a secondinput D, and D,, respectively, of the other gate and are furthermoreconnected via the junction points P and P, to the control members R, andR of the amplifying channels.

The gate circuits ND, and ND, may be formed by Nand-circuits so that theoutput voltage is low only when the voltage at thetwoinputs is high.Thus, the voltage at the output is high whenthe voltage at just one ofthe inputs is low. The control members R, and R, are arranged so thatwhen the voltage at the points P and P.,, respectively, is high, thecontrol members are cut ofi or at least the amplification in theamplifying channels is so low that acoustic feedback cannot occur. Inprinciple, the operation of the arrangement is of course not dependentupon a given polarity of the various signal voltages, neitherindividually nor in relationship, so that it will be obvious that thegates may be arranged differently and, if desired, voltages may beinverted by means of inverters, for example,

the control voltages applied to the control members R, and R,, providedthat the arrangement as a whole continues performing the same function.

Apart from a given degree of frequency dependence, if any, the signallevels at the various inputs of the comparison devices K, and K, areproportional to the signals delivered by the corresponding microphones,independently of the state of the control members R, and R,.

In the rest position of the arrangement the microphones do not deliver avoltage and the signal levels at the inputs of the comparison devices K,and K, are low (zero level) so that the voltage of A, is not higher thanthat of B, and that of A, is not higher than that of B,. The voltages atthe inputs C, and C, of the gates ND, and ND, will be low and thevoltages at point P and P will be high. The control members R, and R,are then cut off.

When someone talks into the microphone M, the signal produced by themicrophone is amplified by the amplifiers T,, FK, and FR, and rectifiedby the rectifiers DA, and DB, so that the signal level at the inputs A,and B, increases. In known manner, the delay device S, is designed sothat the rise in the voltage level at A, is substantially not delayed.The voltage at the input C, remains low, but that at the input A,exceeds that at the input B, so that a high voltage appears at bothinputs C, and D, of the gate ND, and the voltage at point P, becomeslow. As a result, the amplifying channel between the microphone M, andthe loudspeaker H, is opened through the control member R,.

The loudspeaker H, is located in the same room as the microphone M, andpreferably is mounted in the same cabinet as the microphone so that thedistance between these two elements is small. It is desirable for thesound signal delivered by the loudspeaker H, to be as high as possible,preferably at the normal speech level. Therefore, the microphone M, alsoreceives a very strong sound signal which, owing to the nearness of theloudspeaker H,, may be many times stronger than the sound received bythe microphone M,. In a practical case, as a result of the directcross-talk between the loudspeaker H, and the microphone M,, thestrength of the signal at point P, may be times that of the signalatrpoint P,. This cross-talk signal is amplified via the amplifiers T,,FR, and FK, and subsequent to rectification by the rectifiers DB, andDA, it is applied to the inputs B, and A, so that the signal level atthese inputs rises. However, owing to the acoustic delay between theloudspeaker H, and the microphone M, the rise of the signal voltage atthe inputs B, and A, will take place somewhat later than at the inputsA, and B, and, more particularly, not before the voltage at point P, hasbecome low.

The ratio between the amplification by the amplifiers PK, and F K, onthe one hand and that by the amplifiers FR, and FR, on the other hand ischosen so that it slightly exceeds the ratio between the signalsappearing at the points P, and P, when the microphone M, is talked into.When the signal strength at point P, is 10 times that at point P,, thesignal amplification by the amplifiers FK, and FK, may be 1 1 times thatof the amplifiers FR, and FR,.

As a consequence the signal level at the input B, remains slightly lowerthan that at A, so that the low voltage at point P, is maintained.

On the other hand, since the amplification by the amplifier FK, is muchlarger than that by the amplifier FR,, the signal at the input A, willbe much stronger (for example, more than 100 times in the present case)than the signal at point 8,. However, as a result of the aforesaidacoustic delay the voltage at the input C, cannot rise to a high valuebefore the voltage at the input D, of the gate ND, has assumed a lowvalue. As a result, the gate ND, is closed and hence the signal at theinput A, cannot have any effect or, in other terms, the amplifyingchannel between the microphone M, and the loudspeaker H, remains openand that between the microphone M, and the loudspeaker H, remainscutoff.

During the short intervals between the words this situation must notvary, at least not as long as the other party at the microphone M, doesnot want to interrupt. It should be noted that as a result of theacoustic delay the signal at point P, not only appears somewhat laterthan at point P,, but also disappears somewhat later. Owing to theeffect of the delay device 8,, which discharges comparatively slowly,the high signal level at the input A, is maintained for a short timeafter the signal at point P, has disappeared, i.e., for a timesufficient to insure that the signal at point P, also has decayed. Theparty in the other room may interrupt the first one by supplying anadequate sound signal to the microphone M,. Owing to the combined effectof this sound signal and. of the cross-talk signal from the loudspeakerH,, the signal level at the input B, can rise above that at the inputA,. As a consequence the voltage at the input C, of the gate ND, dropsso that a high voltage appears at point P resulting in the cut-off ofthe control member R,. The voltages at the two inputs of the gate ND,are now both high. The point P, then has a low voltage so that on theone hand the gate ND, is closed and on the other hand the control memberR, opens the amplifying channel between the microphone M, and theloudspeaker H,,. This change-over is performed very smoothly.

As stated above, the signal level at the input B, initially was slightlylower than the signal level at the input A,. It is desirable for thisdifference to be as small as possible. This means that the microphone M,requires only a little additional signal for causing the signal level atB, to exceed that of A, or, in other words, the speaker at themicrophone M, need raise his voice only to a slight extent to gainaccess to the device by reversing the direction of transmission of theamplifier.

Cross-talk from the loudspeaker H, to the microphone M, is in generalcomparatively intimately dependent upon frequency. When conventionalloudspeakers and microphones are employed, cross-talk will be strongestat frequencies appearing at the central region of the speech spectrum,for example, between 1,000 Hz and 1,500 Hz. The ratio between theamplifications of PK, and FR, can then be adjusted in the optimum caseso that at frequencies at which cross-talk is a maximum the signal levelat the input B, still is slightly lower than that at the input A,. Athigher and lower frequencies the level difference may then beconsiderably greater, which means that at these frequencies acomparatively strong signal has to be supplied to the microphone M, inorder to cause the level at input B, to exceed that at A,. This is notdesirable because it is quite possible that frequencies at which maximumcross-talk occurs are only weakly represented in the speech spectrum ofthe speaker.

In order to mitigate this disadvantage frequency correction may becarried out in the amplifiers so that the transmission through theamplifier FK, exhibits the same frequency depen dence as thetransmission of the signal through the control member R,, the amplifierU,, the loudspeaker H,, the microphone M,, the amplifier T, and theamplifier F R,.

A correction may be carried out, for example, only in the amplifiers FR,and FR, so that the amplification of the high and low frequenciesexceeds that of the medium frequencies. As an alternative, a correctionmay be carried out only in the amplifiers FK, and FK, so that the lowand high frequencies are amplified to a lesser extend than the mediumfrequencies. Even a mixed correction in the four amplifiers is possible.By means of this frequency correction it may be achieved that despitecross-talk the speech level of the interrupting party at the microphoneM, need exceed the level at the microphone by only a small amount, forexample, by 5 db.

When the loudspeakers and the microphones are arranged in the samecabinets, the cross-talk level will be high. This is only an imaginarydisadvantage. There is also an advantage in that direct cross-talkexceeds by far indirect cross-talk due to reflections from nearbyobjects or echos in the room. The overall cross-talk is therefore apractically constant factor and exhibits very little dependence upon theincidental disposition of the loudspeaker microphone combination in theroom. This permits an optimum adjustment of the amplifiers FK,, FR,,etc.

As stated above, in principle, in the rest condition the control membersR and R may be adjusted so that they either completely cut off theamplifying channel or so that a small steady amplification is left. Ithas been found that the latter alternative is to be preferred because inthat case the distance between the rest condition and the completelyopen condition of the channel is smaller so that the opening of achannel from the rest condition is performed more smoothly or, in otherwords, even if the strength of the first syllable spoken iscomparatively low, the relevant amplifying channel is immediately openedso that no part of this syllable is clipped.

However, some degree of transmission will then take place also throughthe channel not conveying speech which is not desirable in view of thefact that due to cross-talk the sound level at the microphone of saidchannel is much higher than at the microphone of the open channel. Thismay give rise to acoustic feedback in the system.

In order to avoid this phenomenon, auxiliary damping or control membersDR and DR may be included in the amplifying channels. These auxiliarycontrol members are controlled by the voltages at points P and P that isto say by the control voltages which govern the control members in theother channel. The damping members DR and DR are arranged so that theyare conducting in the rest condition of the arrangement, that is to say,in the example described above, when there is a high voltage at thepoints P and P When speech is transmitted through the amplifying channelbetween the microphone M and the loudspeaker 1-1,, the voltage at pointP, remains high so that the damping member DR continues to conduct.However, under the control of the low voltage at point P the dampingmember DR completely cuts off the other amplifying channel.

What is claimed is:

l. A speech-controlled bidirectional amplifier comprising first andsecond amplifying channels each of which includes a transmission controlmember governed by a control voltage derived from the speech signalsappearing in the two amplifying channels at control points preceding thecontrol member of each channel, first and second comparison devices eachhaving first and second inputs and designed so that they supply anoutput work signal when the signal level at the first input exceeds thesignal level at the second input, means for applying the speech signalsof each channel individually through a pair of first signal circuits tothe first input of the two comparison devices and individually through apair of second signal circuits to the second input of the two comparisondevices, the amplification of the first two signal circuits beingarranged to exceed that of the second two signal circuits, a thirdcomparison device including first and second gate circuits each havingfirst and second inputs, means for individually applying said outputwork signals to the first input of said first and second gate circuitsrespectively, means for coupling the output of each gate circuit withthe second input of the other gate circuit in a manner such that a worksignal applied to one of the first inputs of one of the gate circuitswill cut off the other gate circuit, the control voltages for the twocontrol members being derived from the outputs of the gate circuits ofthe third comparison member.

2. An amplifier as claimed in claim 1, characterized in that the firstand/or second signal circuits include frequency-correction circuitsarranged to produce a uniform frequency response over a wide range ofaudio frequencies.

3. An amplifier as claimed in claim 1 characterized in that the twofirst signal circuits each include a charging circuit for maintainingthe signal voltages appearing at the first inputs of the first andsecond comparison members for a given time after the speech signalapplied to the respective signal circuit has terminated.

4. An amplifier as claimed in claim 1, characterized in that the controlmembers are adjusted to a control level such that in the rest conditionof the amplifier each channel is partially open and exhibits arelatively low amplification, each amplifying channel further includinga second control member governed by the control voltage developed at theoutput of the gate circuit of the other channel so that when one channelis open in response to a speech signal the other channel is completelycut ofi.

5. A voice-operated bidirectional amplifier comprising first and secondtransmission channels including first and second transmission controlmembers, respectively, each controlled by speech signals appearing at acontrol point in each channel that precedes the respective channelcontrol member, first and second comparison devices each having firstand second inputs and designed to compare the signal levels appliedthereto so as to produce a first output signal when the signal level atthe first input exceeds the signal level at the second input, a firstpair of signal coupling circuits for individually coupling the speechsignals at the control points of said first and second channels to thefirst input of said first and second comparison devices, respectively, asecond pair of signal coupling circuits for individually coupling thespeech signals at the control points of said first and second channelsto the second input of said first and second comparison devices,respectively, a third comparison device including first and second gatecircuits each having first and second inputs, means for coupling theoutput of said first and second comparison devices to the first input ofsaid first and second gate circuits, respectively, means for couplingthe output of the first gate circuit to the second input of the secondgate circuit and to a control terminal of said first control member, andmeans for coupling the output of the second gate circuit to the secondinput of the first gate circuit and to a control terminal of said secondcontrol member, said first, second and third comparison devices beingarranged so that in the rest condition of the amplifier with no speechsignals present each gate circuit supplies a first control voltage toits respective control member that conditions same to provide highsignal attenuation and simultaneously primes the second input of theother gate circuit so that the first channel to receive a speech signalwill switch its respective gate circuit to provide a second controlvoltage that conditions its respective control member to provide lowsignal attenuation and simultaneously blocks the operation of the othergate circuit.

6. An amplifier as claimed in claim 5 wherein said first and secondpairs of coupling circuits are arranged to transfer signals to theinputs of said first and second comparison devices that are proportionalto the signal levels appearing at the respective channel control pointsso that the comparison devices can be switched in the middle of a speechtransmission by an increase in the level of the signal coupled to thesecond input of the operative one of said first and second comparisondevices such that the signal level at the second input thereof exceedsthe signal level at the first input thereof.

7. An amplifier as claimed in claim 6 wherein said signal couplingcircuits are adjusted so that the amplification of the first pair ofsignal coupling circuits exceeds the amplification of the second pair ofsignal coupling circuits.

8. An amplifier as claimed in claim 5 further comprising signal delaymeans connected in series with the first inputs of said first and secondcomparison devices.

9. An amplifier as claimed in claim 8 wherein said first and secondpairs of coupling circuits are arranged to transfer signals to theinputs of said first and second comparison devices that are proportionalto the signal levels appearing at the respective channel control pointsand independently of the attenuation condition of said first and secondcontrol members.

1. A speech-controlled bidirectional amplifier comprising first andsecond amplifying channels each of which includes a transmission controlmember governed by a control voltage derived from the speech signalsappearing in the two amplifying channels at control points preceding thecontrol member of each channel, first and second comparison devices eachhaving first and second inputs and designed so that they supply anoutput work signal when the signal level at the first input exceeds thesignal level at the second input, means for applying the speech signalsof each channel individually through a pair of first signal circuits tothe first input of the two comparison devices and individually through apair of second signal circuits to the second input of the two comparisondevices, the amplification of the first two signal circuits beingarranged to exceed that of the second two signal circuits, a thirdcomparison device including first and second gate circuits each havingfirst and second inputs, means for individually applying said outputwork signals to the first input of said first and second gate circuitsrespectively, means for coupling the output of each gate circuit withthe second input of the other gate circuit in a manner such that a worksignal applied to one of the first inputs of one of the gate circuitswill cut off the other gate circuit, the control voltages for the twocontrol members being derived from the outputs of the gate circuits ofthe third comparison member.
 2. An amplifier as claimed in claim 1,characterized in that the first and/or second signal circuits includefrequency-correction circuits arranged to produce a uniform frequencyresponse over a wide range of audio frequencies.
 3. An amplifier asclaimed in claim 1 charaCterized in that the two first signal circuitseach include a charging circuit for maintaining the signal voltagesappearing at the first inputs of the first and second comparison membersfor a given time after the speech signal applied to the respectivesignal circuit has terminated.
 4. An amplifier as claimed in claim 1,characterized in that the control members are adjusted to a controllevel such that in the rest condition of the amplifier each channel ispartially open and exhibits a relatively low amplification, eachamplifying channel further including a second control member governed bythe control voltage developed at the output of the gate circuit of theother channel so that when one channel is open in response to a speechsignal the other channel is completely cut off.
 5. A voice-operatedbidirectional amplifier comprising first and second transmissionchannels including first and second transmission control members,respectively, each controlled by speech signals appearing at a controlpoint in each channel that precedes the respective channel controlmember, first and second comparison devices each having first and secondinputs and designed to compare the signal levels applied thereto so asto produce a first output signal when the signal level at the firstinput exceeds the signal level at the second input, a first pair ofsignal coupling circuits for individually coupling the speech signals atthe control points of said first and second channels to the first inputof said first and second comparison devices, respectively, a second pairof signal coupling circuits for individually coupling the speech signalsat the control points of said first and second channels to the secondinput of said first and second comparison devices, respectively, a thirdcomparison device including first and second gate circuits each havingfirst and second inputs, means for coupling the output of said first andsecond comparison devices to the first input of said first and secondgate circuits, respectively, means for coupling the output of the firstgate circuit to the second input of the second gate circuit and to acontrol terminal of said first control member, and means for couplingthe output of the second gate circuit to the second input of the firstgate circuit and to a control terminal of said second control member,said first, second and third comparison devices being arranged so thatin the rest condition of the amplifier with no speech signals presenteach gate circuit supplies a first control voltage to its respectivecontrol member that conditions same to provide high signal attenuationand simultaneously primes the second input of the other gate circuit sothat the first channel to receive a speech signal will switch itsrespective gate circuit to provide a second control voltage thatconditions its respective control member to provide low signalattenuation and simultaneously blocks the operation of the other gatecircuit.
 6. An amplifier as claimed in claim 5 wherein said first andsecond pairs of coupling circuits are arranged to transfer signals tothe inputs of said first and second comparison devices that areproportional to the signal levels appearing at the respective channelcontrol points so that the comparison devices can be switched in themiddle of a speech transmission by an increase in the level of thesignal coupled to the second input of the operative one of said firstand second comparison devices such that the signal level at the secondinput thereof exceeds the signal level at the first input thereof.
 7. Anamplifier as claimed in claim 6 wherein said signal coupling circuitsare adjusted so that the amplification of the first pair of signalcoupling circuits exceeds the amplification of the second pair of signalcoupling circuits.
 8. An amplifier as claimed in claim 5 furthercomprising signal delay means connected in series with the first inputsof said first and second comparison devices.
 9. An amplifier as claimedin claim 8 whereiN said first and second pairs of coupling circuits arearranged to transfer signals to the inputs of said first and secondcomparison devices that are proportional to the signal levels appearingat the respective channel control points and independently of theattenuation condition of said first and second control members.